Enclosed work cabinets for laboratories employing exhaust fans for removing undesirable and dangerous fumes are well known in the laboratory construction industry. An example of such a fume hood is shown in U.S. Pat. No. 4,534,281 which is owned by the Applicant herein.
Prior art fume hoods generally comprise a cabinet which defines an enclosed working area. The cabinet has an opening for providing access to the working area, and typically includes a door movable between open and closed positions to provide selective access to the working area. The prior art fume hoods also include an exhaust system, which is typically connected to the top of the cabinet, for venting fumes that collect in the working area of the cabinet.
Such prior art fume hoods also typically incorporate a bypass located in the face of the cabinet above the opening to the cabinet. The bypass allows air to be drawn through the fume hood via the bypass when the door to the fume hood is moved from the open to the closed position. Thus, the bypass maintains the face velocity of the fume hood at a substantially constant value.
A problem associated with the prior art fume hoods concerns the undesirable interaction between the door to the fume hood and the bypass. Prior art doors typically comprise a single panel which moves vertically upward from the closed to the fully open position. One problem associated with such an arrangement is that the bypass becomes blocked off completely after the door is moved only a short distance from the closed toward the open position. To provide convenient access to the working area, the door must be moved further in the upward direction, thereby increasing the size of the opening to the working area. However, since the bypass is blocked by the door panel, further upward movement decreases the face velocity of the fume hood and presents a threat that toxic fumes will escape the fume hood through the opening, thereby endangering persons working in the vicinity of the fume hood in the laboratory.
One solution to this problem is to move the bypass farther upwardly on the face of the fume hood so that more upward door movement is required to fully cover the bypass. However, such a design requires an increase in the height of the fume hood, and many laboratories do not have sufficient ceiling height to accommodate taller fume hoods. Likewise, the single door panel construction typically does not allow the door panel to be raised to the fully open position because the door contacts the ceiling of the laboratory before the fully open position is reached. Thus, complete access to the working area of the fume hood cannot be obtained in many laboratories with a single door panel which moves vertically upward.
And still a further problem associated with the prior art fume hoods is that in some circumstances, it is desirable to adjust the size of the bypass opening due to particular venting needs or changes in laboratory conditions.